The invention relates to a method of feeding a fluorescent lamp, wherein the actual power through the lamp is measured, an actual power value is compared with a target power value, and, in the case of a significant difference, the power sent through the lamp is adapted.
Such a method is disclosed in U.S. Pat. No. 5,952,793. The light output of a fluorescent lamp, such as a TL lamp, is also determined by the power flowing through such a lamp. This power must be controlled by a ballast, i.e. a power supply that makes sure that the power through the lamp is stabilized. The power through the lamp depends to a substantial degree on many factors, such as the lamp type, the temperature, the condition of the lamp and the lamp electrodes, etc. Therefore, use is made of a control circuit that enables the right amount of power to be accurately sent through the lamp, and the actual power through the lamp is continuously measured by means of an analog-to-digital (A/D) converter, and, in the case of a deviation from the target power, the power sent through the lamp by the ballast is adapted. Frequently, such a ballast comprises dim means that are capable of setting the target power value.
A drawback of the known method resides in that during measuring the actual power by means of said A/D converter, peaks and other irregularities occur, which are not visible but which may cause the control by the ballast to become unstable. This problem is solved in known manner by filtering the analog signal by means of various filters before the signal is sampled. A drawback of this solution is that it causes the response time of the system to be slowed down. In addition, different signals require different filters, so that the hardware has to be adapted continually. Besides, filters in the form of hardware are voluminous and comparatively expensive.
It is an object of the invention to provide an inexpensive, efficient method and ballast for feeding a fluorescent lamp, said method and said ballast enabling a short response time to be achieved and/or being capable of being flexibly employed for different lamp types and under different conditions.
To achieve this, the actual power value is determined by a moving weighted average of a series including the last-measured actual power values, a measured actual power value being substituted with a replacement value if said measured actual power value exhibits a deviation relative to the average power value that exceeds a predetermined maximum deviation. Therefore, instead of filtering the analog signal, a correction is made in the digital samples originating from the A/D converter. If the value of a sample deviates more than a predetermined percentage, for example 10%, from the (weighted) average of the series of samples last taken, then this value is substituted with a replacement value. Preferably, this replacement value is equal to the closest, predetermined, maximum deviating value, for example the average value plus or minus 10%. In this manner, the influence of short-lived peaks in the signal is moderated and a digital solution is offered that is flexible, because it is programmable, and that enables a shorter response time than analog filters. The average may be an ordinary average of the last series of measured values, however, it is alternatively possible to assign more weight to the most recently measured values.
Preferably, if the target power value changes, the predetermined maximum is temporarily increased until the actual power has approximated the new target power value. By virtue thereof, a quick response by the lamp is possible when the user changes the dim setting. If the maximum is set to xe2x80x9cinfinitexe2x80x9d, this means that correction of peaks in the signal does not take place at all. And anyway peak correction is not necessary as in the case of a new dimmer setting, a stable light output of the lamp is temporarily less important.
Preferably, the predetermined maximum deviation can be adjusted in dependence on the target power value. This is important, particularly, in the case of a low target power value. Let us assume, for example, that the power may have a digital value in the range between 0 and 255 (1 byte). If the maximum deviation is defined as a percentage (for example 10%) of the average value, then the problem arises that in the event of a low average value (in this case below 10), the maximum deviation is smaller than the smallest possible digital representation, i.e. the number 1. Therefore, the maximum deviation must at least be set to (digital) 1.
The invention also relates to a ballast for feeding a fluorescent lamp, which ballast comprises a control circuit for controlling the power through the lamp, which control circuit includes sampling means capable of measuring the actual power through the lamp, processor means capable of determining an actual power value by calculating a moving weighted average of a series of last-measured actual power values, and capable of substituting a measured actual power value with a replacement value if said measured actual power value exhibits a deviation in excess of a predetermined maximum deviation from the average value, and said processor means also being capable of comparing an actual power value with a target power value, and of adapting the power sent through the lamp in the case of a significant difference.
These and other aspects of the invention will be apparent from and elucidated with reference to an exemplary embodiment.